The separation membranes used in water treatment units are polymer membranes for water treatment with the objective of purifying water (for example, removing turbidity from river water and groundwater, clarification of industrial water, and for the treatment of wastewater and sewage). Various types of polymers are used in the constitution of polymer membranes for water treatment. For example, various polymeric materials such as polysulfones (PS), polyvinylidene fluoride (PVDF), polyethylene (PE), cellulose acetate (CA), polyacrylonitrile (PAN), and the like are used for separation membranes in polymer membranes for water treatment.
Meanwhile, as the morphology of a separation membrane used in water treatment units, porous hollow fiber membranes are used.
Examples of the properties that are required in a polymer membrane for water treatment, in addition to the goal of separation properties, include having superior water permeability and superior physical strength, high stability toward a variety of chemical substances (namely, chemical resistance), superior hot water resistance, and the like.
For example, cellulose acetate separation membranes are known not to become contaminated even over long-term use, with relatively high water permeability (for example, Patent Document 1).
However, its mechanical strength is low, and furthermore its chemical stability is inadequate. Consequently, when the separation membrane becomes contaminated, cleaning using physical or chemical means such as chemical products are extremely problematic.
Moreover, hollow fiber membranes made from poly(vinylidene fluoride) resin have been proposed as polymer membranes for water treatment having both superior physical strength and chemical resistance (for example, Patent Document 2).
However, poly(vinylidene fluoride) separation membranes tend to become contaminated progressively more readily with use.
Furthermore, hollow fibers made of vinyl chloride resin (for example, see Patent Document 3) have been proposed as separation membranes that can accomplish highly versatile water treatment methods.
However, polymer membranes for water treatment decrease the pure water permeability when a capacity for removing relatively small impurities (i.e., a filtration capacity) increases to the so-called ultrafiltration level. Conversely, when the pure water permeability is increased, the filtration capacity is relatively lower compared to an ultrafiltration membrane and reaches the level of a microfiltration membrane. In this way, there is a trade-off relationship between the filtration capacity and the pure water permeability, so that there is the problem with large scale facility that are required to maintain both a high filtration capacity and sufficient water permeability.
In addition, the polymer membranes for water treatment manufactured from vinyl chloride resins, polyethylene or the like have problems that are inferior to other resins in terms of strength, and furthermore that substantially decrease the water permeability due to thermal deformation under high temperature conditions.